Polymeric gas-separation membranes are well known and are in use in such areas as production of oxygen-enriched air, production of nitrogen from air, separation of carbon dioxide from methane, hydrogen recovery from various gas mixtures and removal of organic vapors from air or nitrogen.
The preferred membrane for use in any gas-separation application combines high selectivity with high flux. Thus, the membrane-making industry has engaged in an ongoing quest for polymers and membranes with improved selectivity/flux performance. Many polymeric materials are known that offer intrinsically attractive properties. That is, when the permeation performance of a small film of the material is measured under laboratory conditions, using pure gas samples and operating at modest temperature and pressure conditions, the film exhibits high permeability for some pure gases and low permeability for others, suggesting useful separation capability.
Unfortunately, gas separation in an industrial plant is seldom so simple. The gas mixtures to which the separation membranes are exposed may be hot, contaminated with solid or liquid particles, or at high pressure, may fluctuate in composition or flow rate or, more likely, may exhibit several of these features. In gas mixtures that contain condensable components, such as C3+ hydrocarbons, it is frequently the case that the mixed gas selectivity is lower, and at times considerably lower, than the ideal selectivity. The condensable component, which is readily sorbed into the polymer matrix, swells or, in the case of a glassy polymer, plasticizes the membrane, thereby reducing its selective capabilities.
A good example of these performance problems is the separation of hydrogen from mixtures containing hydrogen, methane and other hydrocarbons in refineries and petrochemical plants. The principal technologies available to recover hydrogen from these gas mixtures are cryogenic separation, pressure swing adsorption (PSA), and membrane separation. Membrane gas separation, the newest, is based on the difference in permeation rates of gas components through a selective membrane. Many membrane materials are much more permeable to hydrogen than to other gases and vapors. One of the first applications of gas separation membranes was recovery of hydrogen from ammonia plant purge streams, which contain hydrogen and nitrogen. This is an ideal application for membrane technology, because the membrane selectivity is high, and the feed gas is clean (free of contaminants, such as heavier hydrocarbons). Another successful application is to adjust hydrogen/carbon monoxide or hydrogen/methane ratios for synthesis gas production. Again, the feed gas is free of heavy hydrocarbon compounds.
Application of membranes to refinery separation operations has been much less successful. Refinery gas streams contain contaminants such as water vapor, acid gases, olefins, aromatics, and other hydrocarbons. At relatively low concentrations, these contaminants cause membrane plasticization and loss of selectivity. At higher concentrations they can condense on the membrane and cause irreversible damage to it.
When a feedstream containing such components and hydrogen is introduced into a membrane system, the hydrogen is removed from the feed gas into the permeate and the gas remaining on the feed side becomes progressively enriched in hydrocarbons, raising the dewpoint. For example, if the total hydrocarbon content increases from 60% in the feed gas to 85% in the residue gas, the dewpoint may increase by as much as 25° C. or more, depending on hydrocarbon mix.
Maintaining this hydrocarbon-rich mixture as gas may require it to be maintained at high temperature, such as 60° C., 70° C., 80° C. or even higher, which is costly and may itself eventually adversely affect the mechanical integrity of the membrane. Failure to do this means the hydrocarbon stream may enter the liquid-phase region of the phase diagram before it leaves the membrane module, and condense on the membrane surface, damaging it beyond recovery.
Even if the hydrocarbons are kept in the gas phase, separation performance may fall away completely in the presence of hydrocarbon-rich mixtures. These issues are discussed, for example, in J. M. S. Henis, “Commercial and Practical Aspects of Gas Separation Membranes” Chapter 10 of D. R. Paul and Y. P. Yampol'skii, Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1994. This reference gives upper limits on various contaminants in streams to be treated by polysulfone membranes of 50 psi hydrogen sulfide, 5 psi ammonia, 10% saturation of aromatics, 25% saturation of olefins and 11° C. above paraffin dewpoint (pages 473-474).
Similarly, F. G. Russell, “Operating permeation systems”, Hydrocarbon Processing, Vol. 62, pages 55-56 (1983) recommends that the vapor pressure of water in the gas in contact with the membrane should not exceed 85% of saturation at the temperature of the process, and that the partial pressure of organics such as aromatics, alcohols, ketones and chlorinated solvents should not be more than 10% of the saturation vapor pressure.
A great deal of research has been performed on improved membrane materials for hydrogen separation. A number of these materials appear to have significantly better properties than the original cellulose acetate or polysulfone membranes. For example, modern polyimide membranes have been reported with selectivity for hydrogen over methane of 50 to 200, as in U.S. Pat. Nos. 4,880,442 and 5,141,642.
Unfortunately, these materials remain susceptible to severe loss of performance through plasticization and to catastrophic collapse if contacted by liquid hydrocarbons. Several failures have been reported in refinery applications where these conditions occur. This low process reliability has caused a number of process operators to discontinue applications of membrane separation for hydrogen recovery.
Another example of an application in which membranes have difficulty delivering and maintaining adequate performance is the removal of carbon dioxide from natural gas. Natural gas provides more than one-fifth of all the primary energy used in the United States, but much raw gas is “subquality”, that is, it exceeds the pipeline specifications in nitrogen, C3+ hydrocarbons, carbon dioxide and/or hydrogen sulfide content. About 10% of gas contains excess carbon dioxide.
Membrane technology is intrinsically attractive for removing this carbon dioxide, because many membrane materials are very permeable to carbon dioxide, and because treatment can be accomplished using the high wellhead gas pressure as the driving force for the separation. However, carbon dioxide readily sorbs into and interacts strongly with many polymers, and in the case of gas mixtures such as carbon dioxide/methane with other components, the expectation is that the carbon dioxide at least will have a swelling or plasticizing effect, thereby adversely changing the membrane permeation characteristics.
These issues are again discussed in the Henis reference cited above. Such problems are exacerbated when C3+ hydrocarbons are also present in the stream.
In the past, cellulose acetate, which can provide a carbon dioxide/methane selectivity of about 10-20 in gas mixtures at pressure, has been the membrane material of choice for this application, and about 100 plants using cellulose acetate membranes are believed to have been installed.
Nevertheless, cellulose acetate membranes are not without problems. Natural gas often contains substantial amounts of water, either as entrained liquid, or in vapor form, which may lead to condensation within the membrane modules. However, contact with liquid water can cause the membrane selectivity to be lost completely, by swelling the membrane to the point that it loses mechanical strength and collapses, and exposure to water vapor at relative humidities greater than about 20-30% can cause irreversible membrane compaction and loss of flux. The presence of hydrogen sulfide in conjunction with water vapor is also damaging, as are high levels of C3+ hydrocarbons. These issues are discussed in more detail in U.S. Pat. No. 5,407,466, columns 2-6, which patent is incorporated herein by reference.
Thus, the need remains for membranes that will provide and maintain adequate performance under the conditions of exposure to hydrocarbons, and particularly C3+ hydrocarbons, that are commonplace in refineries, chemical plants, or gas fields.
For many gas separations, to obtain good selectivity requires the use of amorphous glassy polymer materials. These materials, however, tend to be fairly impermeable compared with rubbery or elastomeric polymers, so very thin selective layers are needed to provide adequate transmembrane flux of the permeating gases.
During the early years of gas separation membranes, many problems were encountered in providing a selective layer that was thin, yet free of small pores, pinholes or other defects that would diminish or destroy the separation capabilities. The literature from the 1980s contains many patents from Monsanto and others describing surface treatments or coatings that could heal or seal defects or plug pores to prevent unselective bulk flow of gas. When the treatment or coating was successful, significant increase in selectivity compared with that obtained by the uncoated/untreated base membrane was achieved.
Pioneering patents that describe these efforts include U.S. Pat. No. 3,980,456 to Browall of General Electric, and U.S. Pat. No. 4,230,463 to Henis and Tripodi, of Monsanto. In U.S. Pat. No. 4,230,463, occluding coatings of highly permeable rubbery materials, such as silicone rubber, are applied to enter and plug pores in asymmetric glassy polymer membranes. The unplugged membranes are, in some cases, completely unselective for certain gas pairs. When treated, the plugged membranes exhibit selectivity for the same gas pair that approaches the intrinsic selectivity of the membrane polymer. Other patents that concern sealing of defects in the selective layer or otherwise changing the properties of the selective layer include U.S. Pat. Nos. 4,486,202 and 4,654,055 to Malon et al., of Monsanto, U.S. Pat. Nos. 4,767,422 and 5,131,927 to Bikson et al., of Union Carbide, and U.S. Pat. No. 5,091,216 to Ekiner et al., of Du Pont.
A more recent reference in the same vein is European Patent Application 0 649 676 A1, to L'Air Liquide. This reference takes advantage of a newly developed, high-free-volume (and hence highly permeable) fluoropolymer, Teflon AF®, and discloses post-treatment of gas separation membranes by applying a layer of such material to seal holes or other defects in the membrane surface. A similar approach is taken in U.S. Pat. No. 5,288,304, to Koros and Jones of the University of Texas, which describes the use of polymeric coatings on microporous carbon molecular sieves to plug the pores. In this case, the goal is not to seal defects, but to prevent water, oil or other hydrocarbons from entering the pores of the sieve. No permeation data for gas mixtures containing C3+ hydrocarbons are given.
It is also known to apply a top coating to gas separation membranes to protect the membranes from abrasion or impact during module preparation and in subsequent use, as mentioned in U.S. Pat. No. 4,813,983. This patent also mentions that a top coating layer can protect membranes from liquid contact.
None of the above references addresses the problem of loss of performance caused by plasticization of polymeric membranes when exposed to C3+ hydrocarbon vapors.
U.S. Pat. No. 4,728,345, to Murphy of Monsanto, describes the application of a rubbery polyphosphazene coating to porous separation membranes, following the occluding or pore-plugging approach of '463. The result is again a dramatic increase in selectivity for gas pairs such as hydrogen/methane and carbon dioxide/methane. The patent also claims that the polyphosphazene coating provides stability for the membrane if aliphatic or aromatic hydrocarbons are present in the feed gas. However, no permeation data for gas streams containing such hydrocarbons are given.
Films or membranes made from fluorinated polymers having a ring structure in the repeat unit, in which the fluorinated polymer provides selective capabilities are known. For example:    1. U.S. Pat. Nos. 4,897,457 and 4,910,276, both to Asahi Glass, disclose various perfluorinated polymers having repeating units of perfluorinated cyclic ethers, and cite the gas-permeation properties of certain of these, as in column 8, lines 48-60 of U.S. Pat. No. 4,910,276.    2. A paper entitled “A study on perfluoropolymer purification and its application to membrane formation” (V. Arcella et al., Journal of Membrane Science, Vol. 163, pages 203-209 (1999)) discusses the properties of membranes made from a copolymer of tetrafluoroethylene and a dioxole. Gas permeation data for various gases are cited.    3. U.S. Pat. No. 5,051,114, to Du Pont, discloses gas separation methods using perfluoro-2,2-dimethyl-1,3-dioxole polymer membranes. This patent also discloses comparative data for membranes made from perfluoro(2-methylene-4-methyl-1,3-dioxolane) polymer (Example XI).    4. A paper entitled “Gas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylene” (I. Pinnau et al., Journal of Membrane Science, Vol. 109, pages 125-133 (1996)) discusses the free volume and gas permeation properties of fluorinated dioxole/tetrafluoroethylene copolymers compared with substituted acetylene polymers. This reference also shows the susceptibility of this dioxole polymer to plasticization by organic vapors and the loss of selectivity as vapor partial pressure in a gas mixture increases (FIGS. 3 and 4).    5. U.S. Pat. Nos. 6,361,582 and 6,361,583, co-owned with the present application, describe gas separation processes in which a separation between C3+ hydrocarbons and other gases is performed using composite membranes with fluorinated polymers as the selective layer.